Surface mount technology (SMT), where devices are mounted directly onto a surface without being plugged into it, is widely used in electronic applications. Miniaturization has influenced the usage of SMT, for instance for opto-electronic devices such as light emitting diode (LED) devices. LEDs based on surface mount technology are very small in size and are assembled onto a printed circuit board (PCB) using SMT machines. With the advent of brighter semiconductor materials in LEDs, the usage of surface mount opto-electronic devices has increased (in indoor and outdoor applications) in the area of backlighting, electronic signs/symbols such as variable message signs, or large full color video screens etc.
In bright ambient light conditions, LEDs must be bright enough for better visibility to an observer. The brightness of an LED is dependent on factors such as the type of semiconductor material, the drive current etc. Materials such as Indium Gallium Nitride (InGaN) and Aluminum Indium Gallium Phosphide (AlInGaP) have higher light efficiencies and therefore are used in LED manufacture. For higher drive currents, the LEDs must be assembled in packages that have very low thermal resistance, in order to withstand the heat produced. This, in turn, depends on the overall package design of the LEDs.
Package size is affected by package design. Package size determines the resolution of a display device. Given a fixed assembly area, the larger the package size, the fewer the devices that can be mounted and the lower the display resolution.
During assembly of an opto-electronic device, difficulties will be encountered if the leads of the SMT components do not facilitate proper soldering to the substrate. Presently, SMT devices are designed to have leads with vertical side walls or they extend out from the device so that a proper solder fillet can be formed during solder reflow.
A first example of a known type of surface mount opto-electronic device is shown in FIGS. 1A-1B. FIG. 1A is a top plan view and FIG. 1B is a side elevation.
In a first device 100, an LED 101 is electrically mounted on a contact 102 by way of an electrically conductive medium 103. A bond wire 104 electrically connects the LED 101 to another contact 105. Both contacts 102, 105 extend horizontally through the body 106 of the device 100 (in the orientation shown in FIG. 1A). Most of the body 106 is optically opaque, except for an inverted frusto-conical cavity 107, in which the LED 101, bond wire 104 and inner ends of the two contacts 102, 105 are encased in an optically clear plastic, with the edges of the cavity 107 acting as a reflector for light emitted from the LED 101. The two contacts 102, 105 extend horizontally outwards to the ends of the body 105, wrapping downwards around the lower halves of the sides 108 and back around under the bottom edges 109. During a solder reflow process to secure the device 100 to a substrate 110, the portions of the contacts 102, 105 running down the sides 108 are connected by solder joints 111 to the substrate 110, with the solder joints 111 against the outer vertical sides 108 of the LED device 100.
A second example of a known type of surface mount opto-electronic device is shown in FIGS. 2A-2B. FIG. 2A is a top plan view and FIG. 1B is a side elevation.
In a second device 200, an LED 201 is electrically mounted on a contact 202 by way of an electrically conductive medium 203. A bond wire 204 electrically connects the LED 201 to another contact 205. The whole body 206 of the device 200 is optically clear plastic. The two contacts 202, 205 extend horizontally outwards along the underside of the body 206, ending with horizontal portions outside the body 206. During the solder reflow process the contacts 202, 205 are connected to a substrate 210 by solder joints 211, with the solder joints 211 against the outer vertical surfaces of ends of the contacts 202, 205.
In both the above cases, the soldered points and the electrically conductive members extend outside the edges of the plastic body.
The LED device shown in FIGS. 1A-1B has many disadvantages. For instance, the thermal resistance of the device is very high because the heat path between the LED and the substrate is very long, which in turn, increases the temperature of the LED. Increase in temperature adversely affects the drive current of the LED. The footprint of the device is not much larger than the size of the plastic body “a” by “b” but the solder required to mount it extends well outside that footprint.
The design in FIGS. 2A-2B, with extended electrically conductive members, has an even longer footprint than that of FIGS. 1A-1B. This prevents close assembly of LED devices onto a substrate. The device footprint is length “c”, which is significantly longer than the body ‘a’ of the device. With the solder, the overall required length is even greater.
The required sizes for the prior art devices affects the pixel resolution of an array of LED devices. An array of the devices of FIGS. 2A-2B is shown in FIG. 3, where they are mounted onto a PCB 300. The pitch “d” of the LED devices cannot be reduced. Otherwise short circuits may occur during assembly or use.
Additionally, in the devices of FIGS. 1A-1B and FIGS. 2A-2B, the extended electrically conductive members and solder joints are visibly exposed to an observer and because of their high reflectivity, cause an undesirable disturbance to the eyes. In other words, they reduce the contrast between the LEDs and the substrate. This phenomenon is even more severe in bright ambient light such as sunlight.